Whole House Bypass Humidifiers are Furnace Humidifiers. Bypass Humidifiers or Evaporative Humidifiers introduce moisture into the air by supplying water to a water pad or foam media. Air circulates through the water pad or foam media evaporating the water - humidifying the air. Bypass Furnace Humidifiers come as Flow-through Humidifiers or Drum Humidifiers. The Flow-Through Humidifier type is available as a Self-Powered Bypass Humidifier using an internal fan. Bypass Humidifiers install on the Furnace Plenum or Return Air Supply Duct.